HVDC DC to DC converter with commutating transformer

ABSTRACT

A DC/DC power transformer is provided which is an arrangement for direct transformation of high electric power from one DC voltage level to another DC voltage level without an intermediate AC voltage network. The DC voltage is today basically used for transmission of high electric power at long distances. The DC voltage levels for these transmissions are normally high. The DC/DC power transformer allows several DC voltage levels to be used in one and the same DC voltage network. The principle for this arrangement is that the valve windings (43, 45) from one or several converter transformers (47) are connected to two valve bridges, which generate opposing cyclically variating magnetic flows in the transformer cores (44). One of the valve bridges is operated as an inverter (42) and the other as a rectifier (46) and in this manner the power is transformed from one DC voltage level (U d1 ) to another (U d2 ). At high voltage levels the leakage inductances in the transformers will be high as a consequence of the insulation levels and therefore special arrangements must be made in order to commutate the magnetic energy from one phase of the transformer to another without creating great losses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an arrangement for direct transformation ofelectric power from one DC-(=direct current) voltage to anotherDC-voltage.

2. Description of the Prior Art

In power transmission DC-voltage is used to transmit high electric powerfrom production centers to consumption centers. Since the power isgenerated and distributed with AC networks (1,2) it is necessary totransform the AC-voltage to a DC-voltage (U_(d) in FIG. 1) by means of arectifier (3) and on the other end re-transform the DC-voltage to anAC-voltage by means of an inverter (4). These convertors are composed ofconverter transformers (5, 6) and valves (7) which are connected intovalve bridges (8, 9). The rectifier and the inverter as well as thevalve bridges are known and described in reference 1, chapter 2 and 3.

The rectifier and the invertor can be provided with filters on theAC-voltage side (10, 11) as well as on the DC-voltage side (12, 13).These filters as well as the smoothing reactor (14, 15) on theDC-voltage side are provided in order to filter harmonics in current andvoltage, which are generated as a consequence of the transformation fromAC- to DC-voltage and vice versa. Each rectifier or inverterconsequently needs a lot of equipment which also generates a lot oflosses. This has strongly restricted the utilization of high voltagedirect current as a means of transmitting electric power.

With technology known today relating transformation of electric powerfrom one high voltage DC-voltage level to another high voltageDC-voltage level, the power is converted to an AC-voltage by means of analternator and then converted to the other DC-voltage level by means ofa rectifier. Another known arrangement relates to a series connection ofa couple of converters for increasing or decreasing of the DC-voltagelevel in proportion to the power supplied to or withdrawn from theAC-voltage network (compare reference 2).

Known arrangements of DC/DC transformation for low voltage application(se e.g. chapter 7 in reference 3) are not suitable for powertransmission and high voltage equipment, due to the high requirement forlow noise interference, low losses and high insulation levels, and thehigh leakage inductances in the transformers related to the high voltagelevels.

The known rectifier (3) and inverter (4) are drawn in FIG. 1. In thefigure a 12 pulse configuration is illustrated with star- anddelta-connected converter transformers, which is the most commonconfiguration today. This known configuration and corresponding firingsequence is described in chapter 2.9 of reference 1. In the 12-pulseconfiguration the firing varies cyclically from one valve to another ineach 12-pulse group (8, 9). The two series connected 6-pulse groups ineach rectifier and the inverter are phase shifted 30° since transformervalve windings in the upper group are star connected (16, 18) and in thelower group are delta connected (17, 19). Due to restrictions in maximumpower handling capacity of each transformer unit the transformerwindings may be divided in one, two, three or six units. In each ofthese units there must be at least one AC-winding (20, 21) with the samephase shift as the valve windings in the respective transformer unit.The greatest quantity of transformer units and the lowest power handlingcapacity per unit is achieved if only one valve winding andcorresponding AC winding is placed in one and the same transformer unit.

Since rectifying and inversion with today's power technology isperformed with line commutated valves, the firing and extinction isachieved only with certain firing angle, α, and extinction angle, γ,respectively. Commutation from the valve winding of one phase, to avalve winding of another phase will only be achieved with a certainoverlap angle, u due to the transformer leakage inductance. Due to thesea certain phase shift between the voltage and the current is createdduring the rectification and the inversion processes. This results in adeficit of reactive power as described in reference 2. In order tocompensate for this it has become useful to provide the convertors notonly with ac-filters (10, 11) but also with shunt capacitor banks (22,23) for generation of reactive power. The DC-current control is anessential function of the known DC-voltage transmission. The line directcurrent (I_(d) in FIG. 1) in the known DC voltage transmission iscontrolled by the DC voltages in the converter stations through theformula: ##EQU1## I_(d) =Line DC-current U_(d) ^(R) =DC-voltage inrectifier

U_(d) ^(I) =DC-voltage in inverter

R=Line resistance

The DC-voltages are controlled by the firing and extinction angles andthe tap changers in the way described in chapter 7 of reference 4.

SUMMARY OF THE INVENTION

This invention describes an arrangement and related couplings for directtransformation of electric power from one DC voltage to another. Thearrangement is composed of an inverter bridge (24), convertertransformers (25) and a rectifier bridge (26), in which the firingsequence of the valves (27) is adjusted, so that both bridges generatevariating electromagnetic fields of opposing polarity in the transformercores with related windings.

If the converter transformer in converters having only one valve winding(16, 18, 17, 19) are provided with two galvanically isolated valvewindings (28 resp. 29, 30 resp. 31), on all phases, these valve windingscan be connected to two independent bridges (24 resp 26). Theseindependent valve bridges can now follow two different cyclic firingsequences. By arranging the phase shift between the valve windings fromthe two bridges, opposing magnetic fields can be created in the core(s)of the converter transformer(s) (32, 33). By driving one of the valvebridges as an inverter (25) and the other valve bridge as a rectifier(26), power can be transformed from one DC-voltage level (U_(d1)) toanother (U_(d2)). The relation in voltage and current is thus determinedby the turns ratio in the two valve windings from the two valve bridges.If line commutated valves are used the firing and extinction must beachieved with certain delay angles in relation to an AC referencevoltage winding. An AC voltage winding (34, 35) can be connected to eachphase and transformer unit for this purpose. This winding is connectedto an AC voltage reference net (36), with one bus bar per phase.

The air of this AC-voltage reference net is to form a voltage referenceagainst which the delay angles of the rectifier and inverter bridges arereferred. Sufficient power for the AC voltage reference may beexternally generated or supplied by a three phase synchronous generator(37). The synchronous generator may also by itself or in parallel withshunt capacitor banks (38) provide reactive power corresponding to thereactive power consumption of the line commutated DC/DC powertransformer due to the delay angles. AC-filters (39) may also beconnected to the reference net in order to take care of currentharmonics generated during the rectification and inversion processes.

The principle of the known bridge coupling in that an AC voltageconnected to a transformer core will provide a cyclically variatingmagnetic field. This variating magnetic field generates, through theturns ratio, voltages in the other windings connected to the same core.The cyclic firing sequence in the valve bridges (8, 9) will result in DCvoltages with a certain ripple over the valve bridges. The smoothingreactors (14, 15) inhibit this voltage ripple to pass on to the DClines. When the coupling is provided with a load, current is withdrawn.The current is transformed over the transformer core according to theprinciple of ampere turns ratio balance. This process is explained inchapter 3 of reference 1.

The principle of the invention described herein is that the valvewindings of the inverter bridge (24) may generate a cyclically variatingmagnetic field in a respective transformer core (32, 33). By means of aco-variating the cyclic firing sequence in the rectifier bridge (26) thethus induced voltages will build up a DC voltage (U_(d2)) over therectifier bridge, which generates an opposing magnetic field in thecore. When the rectifier bridge is provided with a load, current will betransformed according to the principle of ampere turns ratio balancebetween primary and secondary windings of a transformer, if selfcommutated vales are utilized, no further windings are needed, since thecyclic sequence with forced commutation will lead to currents beingcommutated from one phase to another according to a predeterminedfrequency.

A self commutated inverter for conversion of high voltage DC to threephase AC is known and described in chapter 6.1 of reference 3. In FIG.6.1 of this reference the inverter is shown. Such a bridge coupling maybe used as inverter bridge (42) in a self commutated DC/DC powertransformer as illustrated in FIG. 4. In the known bridge coupling twoopposing thyristors are always on. In the bridge coupling describedhere, on the contrary, only one thyristor in each three pulse groupshall be on at the same time. Through the cyclic firing and extinctionsequence of the thyristors (G11-G16):

G11→G12→G13→G14→G15→G11→. . . a variating electromagnetic field isgenerated by the valve windings (43) in the transformer windings and itscore (44). The diode valves (D11-D16) commutate the current when anopposite valve has extinguished. When for example the thyristor valveG11 extinguishes the current will continuer through winding A1 due tothe transformer leakage inductance. The voltage in the blocked directionof the thyristor valve will therefore rapidly raise until the diodevalve D14 starts to conduct. A commutating voltage has been built upwhich commutates the current through the winding. The valve windings(43) from such an inverter (42) may be wound on the same transformercore (44) as the valve windings (45) connected to a rectifier (46) builtup of diode valves (D21-D26). Such an inverter generates anelectromagnetic field, which induces voltages in the valve windings (45)of the rectifier. By means of the diode valves (D21-D26) in the bridgecoupling (46) these voltages are rectified.

The firing and extinction sequence of the inverter is designed so thatthe thyristor valves (G11-G16) will be extinguished and fired in acyclic sequence, as shown in FIG. 3, where:

T=Time of a cycle

F=Firing signal

E=Extinction signal ##EQU2## U_(A1), U_(B1), U_(C1) =Phase voltages ininverter valve windings (p.u.)

U_(A2), U_(B2), U_(C2) =Phase voltages in rectifier valve windings(p.u.)

U_(A), U_(B), U_(C) =Phase currents (p.u.)

Two valves connected to different transformer windings are always on,e.g. in time interval ##EQU3## the G11 connected to A1 and the G16connected to B1 are on. The voltage over the inverter bridge (U_(b1))will distribute uniformly over these windings. If we first regard theinterval outside the commutation, i.e. ##EQU4## the whole DC currentwill flow through both valve windings A1 and B1 and be transformed byampere turns ratio balance to valve windings A2 and B2 respectively.Since positive voltage is created in the connection point of A2 andcurrents flows in this direction, the diode valve D21 will conduct inthe forward direction. The diode valve D26 will conduct current intovalve winding B2 from negative polarity. A positive voltage (U_(b2)) isthus created over the rectifier bridge. The commutation process for theself commutated DC/DC power transformer is most easily described by anexample. For example, consider the interval ##EQU5## when the current iscommutated from thyristor vale G16 to thyristor valve G12. G16 isordered to extinguish and a firing signal is emitted to valve G12. Thevalve windings B1 and B2 will continue to conduct current in the samedirection as before due to the leakage inductance of the transformer.The voltage over valve G16 will therefore increase rapidly in theblocking direction, until the diode (D13) of the opposite valve startsto conduct.

Then a voltage is built up over the valve winding opposing the currentflowing through it. This voltage will de-commutate the current throughthe winding. At the same time the positive voltage over valve winding C1will increase the current through this winding with a current derivativedetermined by the relation between applied commutating voltage and thetransformer leakage inductance. When the entire current has commutatedover from B1 to C1 the diode valve D13 extinguishes. Since the currentextinction of the bridge current (I_(b1)) happens almost momentarily,while the current increase is gradual, a saw tooth formed ripple iscreated int he bridge current of the inverter (I_(b1) of FIG. 3). Thesame current changes which have occurred on the inverter valve windingwill also occur in corresponding windings in the rectifier due to theampere turns balance. Here the current only commutates between valves inthe same direction. The bridge current in the rectifier (I_(b2)) willtherefore not present a corresponding ripple.

During the commutation process, the commutating voltages are nottransferred to the rectifier bridge. A six pulse ripple in voltage willtherefore occur in the rectifier (U_(b2) in FIG. 3). In order to isolatethis voltage ripple from the DC-line a smoothing reactor (49) may beplaced inside the DC filter (51).

When the thyristor valve G16 extinguishes and G12 is fired a closedcurrent loop is formed on the rectifier side over the diode valve D26,the valve windings B2 and C2 and diode valve D22. The current will onlyflow in this loop until the valve winding B2 has de-commutated thecurrent to winding C2. Then the diode valve D26 extinguishes. Thus thecommutation is completed. The turns ratio of the self commutated DC/DCpower transformer N₁ :N₂ determines the relation between the bridgevoltages of the rectifier and the inverter U_(b1) :U_(b2) and the bridgecurrents I_(b2) :I_(b1), where

N₁ =number of turns in the valve winding of the inverter (43) and

N₂ =number of turns in the valve winding of the rectifier (45). Theinverter side DC line current, I_(d1), is determined by the averagevalue of the bridge current

    I.sub.d1 =I.sub.b1 /(1-3u.sub.T /T).

The rectifier side DC line voltage, U_(d2), is determined, by theaverage value of the bridge voltage

    U.sub.d2 =U.sub.B2 (1-3u.sub.T /T)).

The effective turn ratio of the DC/DC power transformer is therefore afunction of the load (=U_(d1) :U_(d2) =I_(d2) :I_(d1) =N₁ :N₂ (1-3u_(T)/T). The principle for the firing and extinction of the line commutatedDC/DC power transformer is illustrated in FIGS. 5 and 6. A schematiccircuit diagram for a six pulse group is given in FIG. 5, with thethyristor valves of the rectifier (T21-T26), the valve winding of therectifier (A2, B2, C2), the inverter thyristor valves (T11-T16), theinverter valve windings (A1, B1, C1) and the windings to the AC voltagereference (A3, B3, C3). The letters in these designations determines thephase of respective winding.

The firing sequence of the six pulse group of the line commutated DC/DCpower transformer is shown in FIG. 6. The control pulses for thethyristor valves connected to the inverter are indicated by T11→. . .→T16. The control pulses show the time interval when a valve shall be onand provided with firing pulses as soon as the blocking voltage becomespositive. The currents to the inverter in a star connected valvewindings are denominated I_(A1), I_(B1), I_(C1). The control pulses forthe thyristor valves connected to the inverter are indicated by T21→. .. →T26. The current to the rectifier in delta connected valve windingsare denominated I_(A2), I_(B2), I_(C2). The differences between thesecurrents corrected with respect to turns ratio are the ampere turnsdifferences which each phase of the AC windings must compensate for.These AC currents denominated I_(A3), I_(B3), and I_(C3) are shown inFIG. 6. The AC voltage reference currents may through Fourier analysisbe regarded as a summary of a fundamental component, phase shifted 90°after the voltage and a number of harmonics. The fundamental currentcomponent in the AC reference net represents the reactive power whichmust be provided to the DC/DC power transformer. The harmonics arecompensated by the AC-filters connected to the AC reference net.

The commutation process of the line commutated DC/DC power transformeris illustrated in FIG. 7. The AC voltage reference is exemplified by thephase voltage U_(A3), which in the example in FIG. 5 is in phase withthe phase voltage of the inverter U_(A1). The commutation from valve T12to valve T14 is achieved by the commutation voltage UY_(AlCl) ^(y),which is the difference between the two phase voltages U_(Al) ^(Y) andU_(Cl) ^(Y). U_(Cl) ^(Y) is phase shifted 120° el before U_(Al) ^(Y),while U_(AlCl) ^(Y) (ωt)=U_(Al) ^(y) (sin ωt-sin(ωt+2π/3))=√3 U_(Al)^(Y) sin(ωt-π/6), i.e. U_(AlCl) ^(Y) is phase shifted 30° after U_(Al)^(Y). Commutation from thyristor valve T12 to valve T14 starts withfiring of valve T14 at the instant when an angle (γ+u) remains beforethe phase-to-phase voltage U_(AlCl) ^(Y) becomes zero. The commutationvoltage U_(AlCl) ^(Y) commutes the direct current from valve winding Clto valve winding A1 during the commutation interval "u". The DC-currentthrough winding A1 is demonstrated as I_(A1) ^(Y) in FIG. 7. In theexample in FIG. 5 the valve winding of the rectifier is phase shifted30° before the inverter by means of a Y_(d) 11 coupling. The phasevoltage U_(A1) ^(d) is thus phaseshifted 30° before the AC referencewinding voltage. The phase-to-phase voltage U_(A2C2) ^(d) comes 30°after the phase voltage and is consequentially in phase with the ACreference winding voltage. The commutation from thyristor valve T25 tothyristor valve T21 in the inverter starts α degrees after zero passageof the phase-to-phase voltage U_(A2C2) ^(d) and ends u degrees later.After commutation of the DC current to the valve T21 the DC current inthe rectifier will pass through valve T26 and windings C2 in series withA2 parallel to the winding B2. During this time T21 and T26 conduct onlya third of I_(d2) flows through winding A2. When the valve T26 commutesthe current to the valve T22 the DC-current will pass through winding A2parallel to windings C2 in series with B2. Then the DC-current throughwinding A2 will increase to 2/3I_(d) and so on in the way illustrated bycurrent I_(A2) ^(d) in FIG. 7. The current amplitude I_(A2) ^(d) isexpressed in per unit valve, referring to the AC reference winding side,and is therefore multified with factor √3 as turns ratio factor fordelta connected windings.

The differential current, I_(A3), between rectifier and inverterwindings of phase A, is delivered to the AC reference winding. If thesummary angle (α+γ+u) is the same as the phase shift between these two,this current will have the smooth shape as in FIG. 6. Otherwise currentpeaks will occur in the way demonstrated by current I_(A3) Y_(d) 11 inFIG. 7. In order to minimize current harmonics it is therefore desirableto control the summary angle (α+γ+u) against the same value as the phaseshift between rectifier and inverter valve windings.

In FIG. 1 is shown the known high voltage DC transmission.

In FIG. 2 is shown the proposed arrangement of a 12 pulse line commutateDC/DC power transformer.

In FIG. 3 is illustrated the valve voltages and currents of a selfcommutated sixpulse DC/DC power transformer.

In FIG. 4 is shown the circuit diagram of a sixpulse self commutatedDC/DC power transformer.

In FIG. 5 is shown the circuit diagram of a six pulse line commutatedDC/DC power transformer.

In FIG. 6 is shown the firing and extinction sequence with valvecurrents of a 12 pulse line commutated DC/DC power transformer.

In FIG. 7 is shown the valve and winding currents and voltages of a 12pulse line commutated DC/DC power transformer.

In FIG. 8 is shown the power control strategy of a line commutated DC/DCpower transformer.

In FIG. 9 is shown a physical configuration of the valve windings in athree phase DC/DC power transformer.

In FIG. 10 is shown the circuit diagram of a 12 pulse line commutatedauto DC/DC transformer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The circuit diagram of a six pulse self commutated DC/DC powertransformer is shown in FIG. 4. As already mentioned a voltage ripple iscreated on the rectifier side due to the commutation process. In orderto isolate this voltage ripple from the dc line a smoothing reactor (49)may be connected to the valve bridge and a dc filter (51) outside thesmoothing reactor.

The dc-commutation with the diodes (D11-D16) on the inverter sidegenerates a six pulse ripple in the current (I_(b1) in FIG. 3), whichwith a finite dc capacitor (48) will result in a certain voltage ripplealso over this valve bridge. In order to reduce current and voltageripple the dc capacitor may be designed as a filter. If two six pulsebridges (42) shifted 30° between each other with respect to firing andextinction sequence are series connected the voltage ripple will bereduced and its frequency doubled. The remaining voltage harmonics areabsorbed by the smoothing reactors (49, 50).

FIG. 4 demonstrates that the self commutated DC/DC power transformer canonly transmit power from the inverter side to the rectifier side. Ifpower transmission is desired in both directions the rectifier bridge(46) may be exchanged to another inverter bridge (42), provided withself commutated thyristors and antiparallel connected diodes, where theformer are deactivated during rectifier operation. The powertransmission direction is thereby determined by which of the two bridgesis activated in inverter mode through forced firing and extinctionsequence. The DC-filter (48) is connected to either side of thesmoothing reactor depending upon power flow direction. In principle thevalves in the inverter are built up of self-commutated thyristors(G11-G16) antiparallel connected diodes (D11-D16), which for highvoltages must be series connected and provided with common voltagedivider elements and heat sinks.

A self commutated DC/DC power transformer built up according to theprinciple described here may have normal current control for dc lines asdescribed in chapter 7 and reference 4. The current through thetransformer is determined by the difference in voltage between powersource net and power load net, and pulse frequency, since these affectthe transformer turns ratio N₁ :N₂ (1-3u_(T) /T). The control system ofthe transformer substation should be provided with protections againstblocking by dc line fault and other short circuits which create harmfulovercurrents and overvoltages.

The necessary commutation voltage for the line commutated valves is mosteasily provided by an AC voltage reference winding connected to eachtransformer unit and phase. The AC voltage reference (36) generates asinusoidal flow in the core. The rectifier and inverter bridges settheir delay angles in relation to this reference voltage.

In order to maintain the ampere turns balance in each transformer core(32, 33) the windings in the rectifier and inverter bridges are phaseshifted, so that the valve windings from respective rectifier andinverter bridges simultaneously conduct as much as possible to achievethe best ampere turns balance. As a consequence of the firing,extinction and overlap angles the phase shift between rectifier andinverter can never be completely balanced out on a transformer core. Areactive power deficit occurs that must be compensated by the AC voltagereference winding (34, 35). The necessary reactive power can be suppliedfrom asynchronous generator (36) or via thyristor- and/or breakerswitched shunt capacitor banks, if necessary in combination with SVCcontrol of known technology, described in chapter 10 of reference 4.

In the presently known thyristor valves the firing angle (α) in therectifier is normally controlled between 5° and 20°. In the invertor theextinction angle is normally kept over 17°. Normal values of the overlapangle are 10°-15°. With these delay angles the summary angle (α+γ+u) ofthe DC/DC power transformer becomes between 30° and 60°. With improvedfiring and extinction characteristics the nominal value of the summaryangle (α+u+γ) should be possible to reduce. In the example in FIG. 2 aphase shift between rectifier and inverter of 30° has been chosen. Thisis achieved by couplings Yd11 and Dy11 according to SEN 270101. In orderto achieve greater phase shifts the valve windings of either one or bothsides can be connected in a Z-coupling or with extended delta.

By series connection of six pulse bridges in the rectifier and inverterphase shifted in relation to each other and to the AC windings thecurrent harmonics are reduced in the same manner as for the known 12pulse converter (see reference 1 chapter 8.2), which reduces necessityfor harmonic AC-filters (39). The phase and phase-to-phase voltages anddifferent phase A transformer winding currents of the line commutatedDC/DC power transformer in FIG. 2 are shown in FIG. 7. In this examplethe delay angles are α=10°, γ=15° and u=10°.

The valve winding currents of the six pulse group with Yd11 coupling aredenominated I_(A2) ^(d) in the rectifier and I_(A1) ^(Y) in theinverter.

Corresponding AC reference winding current, I_(A3) ^(Yd11), equals thedifference between these currents. The valve winding currents of theother Dy11-coupled six pulse group are in the rectifier I_(A2) ^(Y) andin the inverter I_(A1) ^(D) and is deduced in the same manner as for thefirst six pulse group. The difference between these currents, I_(A3)^(Dy11), is the corresponding AC reference winding current. The summaryof I_(A3) ^(Yd11) and I_(A3) ^(Dy11) is the total phase current, I_(A3)^(tot), which the AC reference winding shall supply to the DC/DC powertransformer. The fundamental component of this current is the reactivepower current of the DC/DC power transformer phaseshifted 90° after thevoltage. The spikes in this current result because the summary angle(α+γ+u), in this case (=35°), deviates from the phaseshift betweenrectifier and inverter (=30°). In order to achieve a greater phase shiftbetween the valve windings of the rectifier and inverter, the rectifierwindings can be connected into a "Z" or to an extended delta phaseshifter x° before the AC reference winding. The phase shift x can bedesigned freely between 0° and 30°, through choice of number of turnsbetween the delta and star part of the extended delta. The invertervalve winding can at the same time be connected to a delta, phase shift30° after the AC reference winding. In this manner any desired phaseshift between 30° 60° can be obtained. The phase shift between the twoin all other respect equal series connected six pulse groups can beobtained by star-connection of one of the AC reference windings anddelta connection of the other. This procedure is demonstrated for theautocoupled DC/DC power transformer in FIG. 10. In order to minimize thecurrent harmonics the phaseshift between rectifier and inverter should,be chosen so that it coincides with normal stationary operational valueof the summary angle (α+γ+u). The control of the line commutated DC/DCpower transformer may also be adjusted for control against nominal valueof the summary angle. The control system of high voltage DC transmissionis known and described in chapter 7 of reference 4. For a DC/DC powertransformer with lien commutated valves certain additional controlparameters are to be considered due to the influence of the AC voltagereference net. FIG. 8 illustrates schematically the power control of theline commutated CD/CD power transformer (54). The current control ofeach converter is performed in the known manner. The coordinationbetween the different current orders and the voltage of the AC voltagereference net assures that balance is maintained between incoming(P_(in)) and outgoing (P_(ut)) active power. The balance between theconsumption of reactive power (Q) and the generation of reactive power(Qg) provided by AC voltage reference net is controlled by the reactivepower control (53).

The power order (52) is set in one of the stations, for example in therectifier of the supplying dc-network. From here current orders to theentire dc-net are emitted. The power change may not be faster that whatthe reactive power control of the DC/DC power transformer permits. Thecurrent order between supplying net (I_(orderR)) and the supplied net(I_(orderI)) and the inverter of the DC/DC power transformer(I_(order1)) and its rectifier (I_(order2)) are coordinated. Normallycurrent orders are set so that inverters control the voltage, while therectifiers control the current. Increased stability on the AC voltagereference net is achieved i also the current control (55) in theinverter of the supplied ac-network is allowed to control the current(I_(d2)) in the supplied dc-net.

The different windings in the DC/DC power transformer (28-31, 34-35) mayall be placed in one and the same transformer unit. In its most compactform, which probably only is applicable on lower powers, all windingsare placed in one transformer unit. In FIG. 9 such a threephase threephase three winding is shown, for example for windings 30, 31, 35 andcore 33 in FIG. 2. Each transformer core leg (56, 57, 58) has beenprovided with three windings from the same phase. The innermost windings(59A, 59B, 59C) are in this case the three phases (A, B, C) of the ACreference winding (34). The intermediate windings (60A, 60B, 60C) are inthis case the three phases of the inverter valve windings (30) connectedinto a star. The outmost windings (61A, 61B, 61C) are the three phasesof the rectifier valve windings (31), closed in a delta.

The smallest power per unit, which also means the highest number ofseparate units, is achieved if only one valve winding belonging to eachbridge (25 resp 26) and one AC-winding (34, 35) are placed into oneunit. The AC voltage reference net (36) then interconnects the differentAC windings of each transformer unit for each phase.

In reference 2 a form of series and parallel connections of invertersand rectifiers is shown which is a "D.C. voltage transformation in hightension systems".

A rectifier (62) and an inverter (63) are connected in oppositedirections in a DC voltage transformation point so that power istransformed from one DC voltage level (U_(d1)) to another DC voltagelevel (U_(d2)) without sectioning power. If the valve windings of therectifier (64, 65) and of the inverter (66, 67) in these combinedbridges are connected to the same transformer core (68, 69) an autocoupled DC/DC power transformer is achieved as shown in FIG. 10. Therectifier bridge (62) is cascade connected to the inverter bridge (63).With this autocoupling the high voltage valve bridge is designed for thedifference in voltage (U_(d2) -U_(d1)) and the low voltage valve bridgeis designed for the difference in current (I_(d1) -I_(d2)). The resultis that less costly equipment is needed, lower losses are generated andless reactive power is consumed as compared with a more basic concept.

FIG. 10 depicts how the rectifier valve windings (64, 65) are phaseshifted 45° before the inverter valve windings (66, 67) and how at thesame time 30° phase shift is achieved between the two AC referencewindings (71, 71).

The auto coupling is of course also suitable for DC/DC power transformerwith self commutated valves.

REFERENCES

Ref. 1: E. W. Kimbark "Direct Current Transmission", Volume I, 1971 byJohn Wiley & Sons.

Ref. 2: Kanngiesser U.S. Pat. No. 3,942,089 May 1976 "DC VoltageTransformation in High Tension Systems".

Ref. 3: K. Thorborg "Power Electronics", 1988, Prentice-HallInternational (UK) Ltd.

Ref. 4: A. Ekstrom "Kompendium i Hogeffektelektronik", KTH/EKC January1988.

I claim:
 1. An arrangement for transformation of high electric powerfrom one DC voltage level to another DC voltage level, comprising atransformer with a core, a first valve bridge having valves, said firstvalve bridge comprising at least one six-pulse inverter bridge whichincludes a plurality of said self-commutated thyristors and furthercomprising a plurality of diodes, each diode of said plurality of diodesbeing connected antiparallel to a self-commutated thyristor, and asecond valve bridge having valves, said second valve bridge comprising asix-pulse rectifier bridge which includes a plurality of diode valves,one of said first valve bridge and said second valve bridge being arectifier bridge and the other of said first valve bridge and saidsecond valve bridge being an inverter bridge, first valve windings beingwound around said core and connected to said valves of said first valvebridge and second valve windings being wound around said core andconnected to valves of said second valve bridge, said first valvewindings and said second valve windings being galvanically isolated, andmeans for cyclically firing said valves in said first valve bridge andsaid valves in said second valve bridge so that variatingelectromagnetic fields of opposing polarity are generated in said coreby said first valve windings and said second valve windings, said valvesof said first valve bridge comprising self-commutated thyristor valvesconnected to a DC voltage source which supplies DC voltage at said oneDC voltage level, and further wherein a capacitor is provided externalof said first valve bridge and connected between said DC voltage sourceand said self-commutated thyristor valves, whereby said variatingelectromagnetic field generated by said first valve bridge is providedby sequentially firing and extinguishing said self-commutated thyristorvalves, electromagnetic energy stored by said core being commutated fromsaid first valve windings to said second valve windings by saidcapacitor, and voltages and currents induced in said second valvewindings being rectified so that electric energy is transformed to saidanother DC voltage level.
 2. An arrangement according to claim 1 whereinsaid plurality of self-commutated thyristors and said plurality ofdiodes is built up having a common voltage divided and common cooling.3. An arrangement for transformation of high electric power from one DCvoltage level to another DC voltage level, comprising a transformer witha core, a first valve bridge having valves and a second valve bridgehaving valves, one of said first valve bridge and said second valvebridge being a rectifier bridge and the other of said first valve bridgeand said second valve bridge being an inverter bridge, first valvewindings being wound around said core and connected to said valves ofsaid first valve bridge and second valve windings being wound aroundsaid core and connected to valves of said second valve bridge, saidfirst valve windings and said second valve windings being galvanicallyisolated, and means for cyclically firing said valves in said firstvalve bridge and said vales in said second valve bridge so thatvariating electromagnetic field of opposing polarity are generated insaid core by said first valve windings and said second valve windings,said valves of said first valve bridge comprising self-commutatedthyristor valves connected to a DC voltage source which supplies DCvoltage at said one DC voltage level, and further wherein a capacitor isprovided external of said first valve bridge and connected between saidDC voltage source and said self-commutated thyristor valves, wherebysaid variating electromagnetic field generated by said first valvebridge is provided by sequentially firing and extinguishing saidself-commutated thyristor valves, electromagnetic energy stored by saidcore being commutated from said first valve windings to said secondvalve windings by said capacitor, and voltages and currents induced insaid second valve windings being rectified so that electric energy istransformed to said another DC voltage level, wherein said first valvebridge is a first inverter bridge and said second valve bridge is asecond inverter bridge, said first valve bridge and said second valvebridge each comprising self-commutated thyristors, each self-commutatedthyristor being connected antiparallel to a respective diode wherebypower flow is determined by how an in which valve bridge cyclic firingand extinction is applied.